I had a 1980 model. GREAT bike.
But it had 2 big problems.
The swingarm bushing were plastic. No bearing, just a plastic bushing. Mine would have slop in about 100 miles. The dealer couldn't keep them in stock. After a few months they were on perpetual backorder.
The 2nd ( and worst) problem was the shifting drum. They broke easily. The drum was supported on 1 side only. The left side hung in the air. Every kdx 400 I have ever seen had broken at least 1. This was the death of these bikes.